Mass spectrometers can be used in a wide variety of applications in medical, food processing, environmental monitoring, and space exploration. Time-of-flight mass spectroscopy has become the most widely used technique for identifying very large organic molecules. This technique has become the method of choice for most drug discovery and polymer applications. The time-of-flight technique is frequently chosen because it is the only technique capable of the high mass sensitivity needed for many substances.
The time-of-flight mass spectrometry (TOF-MS) technique is a known technique which has seen resurgence in popularity because of cost reductions in electronics and the advent of high temporal resolution detectors. The availability of high temporal resolution detectors has enabled shorter flight tubes to be used, which leads to smaller vacuum systems and lower overall instrument costs. These designs are particularly well suited for use in portable instruments.
Three types of electron multipliers have been used in time-of-flight mass spectrometers (TOF-MS): single channel electron multipliers (SCEM's), discrete dynodes (DD's), and micro channel plates (MCP's). Single channel electron multipliers are no longer used in modern instruments because of their limited temporal resolution (20–30 ns at FWHM) and dynamic range. Discrete dynode electron multipliers exhibit good dynamic range, but are used in moderate and low resolution applications because they provide relatively poor pulse widths (typically, 6–10 ns at FWHM).
MCP-based detectors are used in virtually all high resolution applications because they provide the highest temporal resolution (400 ps at FWHM). In order to preserve the high temporal resolution of MCP-based detectors it is necessary to use a 50 ohm impedance-matched anode and transmission line. Fifty ohm impedance-matched anodes are conical in shape and are typically terminated with an SMA or BNC connector.
In the operation of a typical linear MALDI TOF instrument, analyte molecules, dispersed among matrix material of a sample 11 are ionized by a nitrogen laser 13 as shown in FIG. 1. The resultant ions are held (delayed extraction) and then ejected down a flight tube by the application of high voltage pulses. Mass separation occurs during the flight (typically about 1 meter) to the detector 15, with the lower mass ions 17 arriving first, followed by progressively larger mass ions 19. Upon arrival of an ion at the detector 15, the electron multiplier 21 produces a charge pulse corresponding to the arrival time of each ion as shown by the trace in FIG. 2. A high speed digitizer is then used to record the arrival times of the ions, from which the mass of the ion can be determined.
A second type of time-of-flight instrument utilizes an ion mirror to enable the ions to traverse the flight tube twice, thereby increasing the separation distance (and time) of ions with differing masses. FIG. 3 illustrates a typical reflectron-type time-of-flight mass filter. In operation, ions 31a–31e of various masses are injected into a pusher plate assembly 33 and then ejected orthogonally into the flight tube 35 by the application of a high voltage pulse. The ions then travel to the ion mirror or reflectron lens 37 which reverses their direction and directs the ions to the detector 39 located approximately the same distance from the ion mirror 37 as the ion source. In this arrangement the ions travel approximately twice the distance as in the other types of detectors. Thus, they separate twice as far from each other in time and space without substantially increasing the size of the vacuum system.
A third time-of-flight spectrometer configuration is also known. This geometry, known as co-axial time-of-flight, combines the vacuum chamber simplicity of the linear time-of-flight construction with the enhanced mass resolution provided by the reflectron geometry. FIG. 4 illustrates a coaxial time-of-flight mass spectrometer arrangement. In the coaxial time-of-flight spectrometer, the ions are created behind the detector plate and the microchannel plate and launched into the linear flight tube through center holes in the detector plate and the microchannel plate. A special ion mirror reflects the ions back to the detector. The ion mirror causes the ions to fan out radially in order to impact the active area of the MCP at the end of their return flight.
Despite the simplicity and low cost advantages of the coaxial time-of-flight geometry, instrument designers have largely abandoned this geometry because high temporal resolution detectors could not be produced. MCP based detectors with center holes have been used for scanning electron microscopes (SEMs) and focused ion beam (FIB) applications for many years. Such detectors were also used in early time-of-flight instruments as co-axial TOF detectors. The drawback of the previous design detectors in modern instruments is that the flat metal anodes used to collect the resultant charge from the MCP in response to ion impacts, produced a pulse with a severe ring which lasted several nanoseconds in duration, rendering the known detectors unusable for high resolution TOF mass spectrometry. The detector according to the present invention is a high temporal resolution coaxial time-of flight detector that has been developed to overcome the deficiencies in the known detectors.